Electronic devices, such as flat panel displays and integrated circuits, commonly are fabricated by a series of process steps in which layers are deposited on a substrate and the deposited material is etched into desired patterns. The process steps commonly include plasma enhanced chemical vapor deposition (CVD) processes and plasma etch processes.
Plasma processes require supplying a process gas mixture to a vacuum chamber called a plasma chamber, and then applying electrical or electromagnetic power to excite the process gas to a plasma state. The plasma decomposes the gas mixture into ion species that perform the desired deposition or etch process.
In capacitively excited CVD chambers, the plasma is excited by RF power applied between an anode electrode and a cathode electrode. Generally the substrate is mounted on a pedestal or susceptor that functions as the cathode electrode, and the anode electrode is mounted a short distance from, and parallel to, the substrate. Commonly the anode electrode also functions as a gas distribution plate for supplying the process gas mixture into the chamber. The anode electrode is perforated with hundreds or thousands of orifices through which the process gas mixture flows into the gap between the anode and cathode. The orifices are spaced across the surface of the gas distribution plate so as to maximize the spatial uniformity of the process gas mixture adjacent the substrate. Such a gas distribution plate, also called a diffuser or “shower head”, is described in commonly assigned U.S. Pat. No. 4,854,263 issued Aug. 8, 1989 to Chang et al.
Perforated gas distribution plates typically are rigidly mounted to the lid or upper wall of the plasma chamber. Rigid mounting has the disadvantage of not accommodating thermal expansion of the perforated plate as it acquires heat from the plasma. The consequent mechanical stresses on the plate can distort or crack the plate. Alleviating mechanical stress is most important with the larger distribution plates required to process larger workpieces, such as large flat panel displays. Therefore, a need exists for a gas distribution device that minimizes such thermally induced mechanical stresses.
Another shortcoming of conventional diffusers or gas distribution plates is that they commonly operate at temperatures that are undesirably low and spatially non-uniform. Specifically, while the diffuser receives heat from the plasma in the chamber, a conventional diffuser generally loses heat at its perimeter where it is bolted to the chamber wall or lid. Therefore, the perimeter of the diffuser is significantly cooler than the center, which tends to produce a corresponding undesirable spatial non-uniformity in the surface temperature of the substrate positioned near the diffuser. Furthermore, the heat loss from the diffuser to the chamber wall undesirably reduces the temperature of the diffuser, which can undesirably reduce the substrate temperature.